At present, many electronic products are progressively being made more and more light, thin, power-saving, environment-friendly, and so on. Components in these electronic products may operate under various low DC voltages, such as 3.3 V, 2.5 V, 1.8 V, or the like. Therefore DC-DC converting circuits for converting high DC voltages to low DC voltages are widely used in power supply circuits of many electronic products, such as portable computer systems, liquid crystal display devices, and so on.
Referring to FIG. 5, a conventional DC-DC converting circuit 1 includes a three-terminal regulator 10, a first capacitor 11, a second capacitor 12, and a third capacitor 13. The three-terminal regulator 10 is an AIC1084-18CM type regulator, and includes an input pin 15, a grounding pin 16, and an output pin 17.
The first capacitor 11 has a capacitance of 0.1 μF. Two terminals of the first capacitor 11 are connected to the input pin 15 and the grounding pin 16, respectively. The second capacitor 12 has a capacitance of 0.1 μF. Two terminals of the second capacitor 12 are connected to the output pin 17 and the grounding pin 16, respectively. The third capacitor 13 is an electrolytic capacitor, which has a capacitance of 100 μF and a rated voltage of 16 V. The anode of the third capacitor 13 is connected to the output pin 17, and the cathode of the third capacitor 13 is connected to the grounding pin 16.
In operation, a high DC voltage is supplied to the DC-DC converting circuit 1 via the input pin 15 and converted to a low DC voltage by the three-terminal regulator 10. Then the low DC voltage is outputted via the output pin 17 as an output voltage. The first and second capacitors 11, 12 are used for compensating frequency to prevent the three-terminal regulator 10 from producing high frequency self-oscillation and high frequency noise. The third capacitor 13 is used for reducing low frequency interference at the output terminal 17 when the high DC voltage is supplied.
High precision, minute volume of the three-terminal regulator 10 is commercially available. However, the three-terminal regulator 10 is expensive as an integrated circuit. In such case, the cost of the DC-DC converting circuit 1 may be prohibitive.
What is needed, therefore, is a DC-DC converting circuit that can overcome the above-described deficiencies.